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Portland State University’s Green Building Research Laboratory has completed testing some residential window inserts and reported surprising results. The retrofits saved 20 percent of the pilot homes’ heating bills, double the expectations.

Indow Windows founder and CEO Sam Pardue

Energy models apply the laws of physics, but the hazy laws of human behavior might be behind the surprisingly good results found in recent independent tests of a retrofit window insulation product from Portland, Oregon-based Indow Windows.

“If you do a whole-home weatherization, the average savings is around 30 percent,” says Indow Windows founder and CEO Sam Pardue. “And that’s after spending $20,000 per home. Our inserts cost a quarter of that and deliver a big chunk of the savings. I hope that gets people in the energy efficiency industry to really sit up and take notice.”

What’s interesting about the GBRL test results is that energy models predicted only 10 percent savings over single-pane windows, but Indow Windows in test homes saved 20 percent of the heating energy. How?

Psychology may be the answer. The lab report says a possible explanation for the higher savings is the increased comfort of occupants, which reduces their urge to turn up the thermostat. Without the radiant cooling effect of inefficient glazing, occupants feel warmer even though the room is the same temperature.

The simplicity of an acrylic insert — basically an indoor storm window — belies the complexity of making them cost effective. Window frames in older homes rarely have perfectly square corners; many are skewed by age and settling. An installer must measure every window’s size and shape before ordering Indow Windows for a home. Indow Windows devised laser measuring technologies to capture the precise geometry quickly so it can be transmitted to the factory for custom fabrication.

The full-length version of this article about Indow Windows appears in Energy Priorities Magazine.